This invention relates to semiconductor fabrication processing and, more particularly, to a method for forming active devices for semiconductor structures, such as field effect transistors used in random access memories.
Conventional fabrication techniques used to form the active transistors in memory devices have led to several undesirable results. It has become common practice to form active transistors with spacers on the vertical walls of the transistor gates by first forming disposable spacers and having the oxide spacers in place during conductive doping implantation steps to form the source/drain regions of the transistors. The disposable oxide spacers are eventually removed and replaced with final spacers that possess a desired spacer thickness.
However, during the final spacer etch, when nitride is used as the spacer material, it is difficult to etch the nitride spacer with high selectivity to silicon and oxide and yet insure that all of the nitride is cleared from the source/drain regions of the active transistors. Because of this difficulty, a portion of the field oxide may be removed along with a portion of the silicon substrate that has been implanted with conductive dopants to form the transistor""s source/drain regions. Etching into the field oxide can lead to transistor junction current leakage, while etching into the silicon source/drain region can lead to high source/drain resistance or even open circuits. If either of these conditions occur, they will adversely affect transistor operation.
The present invention discloses a method to form active transistors in a semiconductor memory device that will protect the source/drain region of the active transistors during a spacer etch sequence so as to substantially reduce high source/drain resistance and leakage that may occur in the transistor junction.
Exemplary implementations of the present invention comprise processes for forming active transistors for a semiconductor memory device.
A first exemplary implementation of the present invention utilizes the process steps of forming transistor gates having generally vertical sidewalls in a memory array section and in periphery sections. Conductive dopants are implanted into exposed silicon defined as active area regions of the transistor gates. Disposable (temporary) spacers are formed on the generally vertical sidewalls of the transistor gates. Epitaxial silicon is grown over exposed silicon regions. After the epitaxial silicon is grown, conductive dopants are implanted into the exposed silicon regions to form source/drain regions of the active transistors. The temporary spacers are removed and permanent insulative spacers are formed on the generally vertical sidewalls of the transistor gates.
A second exemplary implementation of the present invention utilizes the process steps listed above, but more specifically, the temporary spacers are formed of oxide and the permanent spacers are formed of nitride.
A third exemplary implementation of the present invention utilizes the process steps of the first exemplary implementation except that the source/drain regions of the active transistor are formed prior to the formation of the epitaxial silicon.